RPD - Retention Flashcards
What is meant by retention?
- A term used to describe the resistance of a denture to lifting away from the tissues (vertical dislodging forces)
In what 3 ways can retention be achieved?
- Mechanical means
- Muscular means
- Physical means
How can retention be achieved mechanically?
- Clasps
- Clasps will eventually fatigue, oral muscles will kick in and help here - as long as the denture is made to accommodate this
How can retention be achieved using muscular forces?
- Shape of the denture
- This comes through time
How can retention be achieved through physical forces?
- Coverage of the mucosa
- Using external forces of adhesion, cohesion, surface tension and atmospheric pressure on the impression surface of the denture.
- Adhesion - surface forces of saliva on denture and mucosa
- Cohesion - surface forces of saliva on denture and mucosa
- Atmospheric pressure - resistance to displacing forces
What is meant by cohesion?
- The sticking together of particles of the same substance
What are the 2 classifications of retention?
- Direct
- Indirect
What is DIRECT retention?
- Resistance to vertical displacement of the denture (clasps, physical forces - oral musculature)
What is INDIRECT retention?
- Resistance to ROTATIONAL displacement of the denture
- Things that prevent the denture from rotating out during function
What are guide planes (direct retention)?
- Two surface on a saddle parallel to each other
- They tend to occur naturally
- As long as you maintain the space, you keep the frictional contact and so make retention longer
Where would you find direct retention?
- Close to the base
- Adjacent to the abutment teeth
- Chances of it being interproximal are unlikely as dental materials tend to be rigid
What is the principal method for providing mechanical retention?
- By using clasps
What is a clasp?
- A metal arm that, when in position contacts the tooth preventing removal of the denture base
To be effective as a retainer, where is a clasp placed on a tooth?
- Below the bulbous part of a tooth (the undercut)
- The bulbous portion of the tooth required identification
- To accomplish this a SURVEY of the cast is performed
What 2 ways can clasps be included in a denture?
- Can be made in wrought metal and incorporated into the denture base
- OR can be included as part of a cast denture base
Where do clasps approach the undercuts from?
- Either the occlusal surface (occlusally approaching) or the gingival margin (gingivally approaching)
Clasps work most efficiently when in conjunction with what?
- When in conjunction with a rest (supporting component)
What is meant by reciprocation in relation to RPD’s.
- As part of the clasp assembly each clasp unit will have an opposing element to the retentive clasp arm to prevent pressure from the clasp acting on the tooth
What is the pattern of retention? (4 points)
- Ideal number of retainers = 3
- Always have one that is going to prevent dislodging
- Want it in the shape of a triangle and want to make the triangle as large as possible
- However, not always possible to create a triangle e.g. in a bilateral free-end saddle
When is the stress relieving clasp system (RPI) used?
- Used in free-end saddle designs to prevent stress on the last abutment tooth
What components make up the stress relieving clasp system (RPI)? (3 points)
Rest (occlusal)
- Mesial of the tooth
- Rounded on the impression surface
Proximal Plate (adjacent to saddle)
- Guide surface of 2-3mm
- Undercut to permit movement
I-bar clasp (gingivally approaching)
- Greatest prominence of the tooth contour
What is the RPI stress relieving clasp system used for?
- Used in free-end saddle designs to prevent stress on the last abutment tooth
What is indirect retention obtained by?
- Obtained by the extension of a partial denture base to provide the fulcrum of a class II lever. (the retainer(s) providing direct retention lie between the fulcrum and that part of the denture which is subject to the displacing force)
What are indirect retainers principally known as?
- Supporting components
What are examples of indirect retainers? (5 points)
- Major connectors
- Minor connectors
- Rest
- Saddle
- Denture base
Is retention required in every adjacent (abutment) tooth to the denture saddle?
- No
What is the minimum amount of clasps and their position to be sufficient depending on the saddle length?
- One clasp on each side of the arch
How long does a tooth have to be to accommodate an Occlusaly approaching clasp?
Greater than or equal 15mm to accommodate the 15mm clasp arm
How long does a tooth a have to be to accommodate a gingivally approaching clasp?
- Does not have to be greater than or equal to 15mm to accommo date the 15mm clasp arm
What is the retentive arm of the clasp?
- A flexible arm which deflects over the bulbosity to engage the undercut of the tooth
What is the reciprocation component of the clasp?
- This component counteracts the force of the retentive component with an equal and opposite force
How much of the clasp can go below the survey line to engage the undercut?
- Only the terminal 1/3 of the clasp
- The other 2/3 seat above the survey line
- Once the terminal 1/3 of the retentive arm has engaged the undercut it should be passive and no longer exert a force on the tooth
Retention of the denture is dependent on how difficult it is to dislodge the clasps. The amount of force required is dependent on what 3 factors?
- Flexibility of the retentive arm
- Placement of the retentive arm
- Depth of the undercut
What is flexibility of the retentive arm of a clasp dependent on? (3 points)
- Material (some materials are more flexible, others are more rigid)
- Length (longer = more flexible)
- Thickness (thinner = more flexible)
What size of gauge do we use for a Co/Cr clasp when analysing undercuts?
- 0.25mm
What size of gauge do we use for a gold clasp when analysing undercuts?
- 0.5mm
What size of gauge do we use for a stainless steel clasp when analysing undercuts?
- 0.75mm
